In a data communication network, digital data among other data, may be-communicated at a data signaling rate from one modem to another modem through a communication media, which may be a leased line of the network or a dial up connection of a general switched telephone network (GSTN), for example. Generally, modems operate at a fixed carrier frequency and a fixed symbol rate and attempt to optimize the data signaling rate based on the conditions of the communication media over which they are communicating. In order to accomplish an optimum data signaling rate, contemporary modems, International Telecommunication Union, Telecommunications Standardization Sector(ITU-T) Recommendation V 0.34, utilize a startup learning procedure before commencing communication during which the modems perform certain predefined start up procedures which include a line probing sequence, for example, to establish the media characteristics over which communication will take place. The current state of the art ITU-T standard for two wire full duplex modems is V 0.34 . An example of a modem employing the V 0.34 standard today includes the Motorola Codex Model 3260 Fast.
Two wire modems for operating in a full duplex mode generally employ an echo canceller to cancel from the received signal any near end and far end echoes resulting from its concurrent signal transmissions. Further, in two wire, full duplex transmission systems, there are system nonlinearities which affect not only the signal transmission, but also both of the near end and far end echoes resulting therefrom. Normally modems do not measure the nonlinearities of the echo signals and, for this reason, cannot provide adequate estimates for preemphasis and transmit power level selection purposes. This measurement is vital for modern modems that are transmitting and receiving at data signaling rates up to 33,600 bits per second. Virtually all known echo cancellers are linear models. This means that any significant non-linearity remains largely uncancelled.
Furthermore the receiver can only resolve non-linear distortion levels to approximately the .mu.-law or A-law companding quantizing noise limits of the PCM Codec (37-39)dB and therefore has no ability to measure the potential improvement in the remote modem's performance if the transmit level was dropped further. As an example, the receiver can not measure the distortion improvement of the remote end when the remote transmitter lowers the transmit power by one additional dB that would result in a non-linear distortion level improvement from 65 dB to 70 dB. The ability to resolve small but still significant differences in -performance will be crucial to all V 0.34 modems and the future proposed ITU-T Recommendation V 0.34 bis modems operating at data signaling rates greater than or equal to 28.8 kbit/s.
In the present V 0.34 recommendation, there is a provision for the transmitter to allow for a power drop as requested by the receiver, with additional capability for the transmitter to drop power within the receiver recommended tolerance. There is also the provision in the V 0.34 recommendation for selecting the transmitter preemphasis filter.
Hence, there is a need for minimizing the effects of non-linearity in the network. The present invention provides a selection of transmitter preemphasis filters and the transmitter output power that minimizes the effects of non-linearity in the network.